This invention relates to seismic survey vessels, and is more particularly concerned with seismic survey vessels suitable for use in performing 3D marine seismic surveys covering large areas.
In order to perform a 3D marine seismic survey, a plurality of seismic streamers, each typically several thousand meters long and containing arrays of hydrophones and associated electronic equipment distributed along its length, are towed at about 5 knots behind a seismic survey vessel, which also tows one or more seismic sources, typically air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones in the streamers, digitised and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed.
At the present time, a typical streamer array used by the Applicant comprises a 700 meter wide array of eight evenly spaced streamers, each about 4000 meters long. The streamers are towed by their lead-ins, ie the armoured electrical cables that convey electrical power, control and data signals between the vessel and the streamers, as described in the Applicant's U.S. Pat. No. 4,798,156, and their spread is controlled and maintained by MONOWING deflectors of the kind described in the Applicant's U.S. Pat. No. 5,357,892.
An array of this relatively large size allows 3D seismic surveys of large areas to be performed very efficiently. However, because the cost of performing such surveys is so high, there is continuous pressure to improve efficiency still further.